The present invention relates to a frequency correction circuit, a radio receiving apparatus, and a frequency correction method, and particularly to a technique for referring to a correlation value of a preamble signal and correcting a frequency offset (frequency deviation) between transmission and reception.
In the field of wireless communication such as mobile phones and wireless LAN (Local Area Network), a receiving apparatus generally detects a periodic symbol timing by carrier sense at the time of starting the reception and also performs subsequent receiving operations according to the detected symbol timing. The receiving apparatus here detects the symbol timing using a preamble signal for synchronous acquisition transmitted in advance of a data signal.
As an example of the communication system that adopts such a preamble signal, Standard ECMA-368, “High Rate Ultra Wideband PHY and MAC Standard”, 3rd Edition, December, 2008 discloses the MB-OFDM (Multiband-Orthogonal Frequency Division Multiplex) system. In order to realize low transmission power and broadband communication which are the characteristics of the UWB (Ultra Wide Band) communication, the MB-OFDM system adopts frequency hopping that performs transmission and reception while hopping among a plurality of frequency bands. The frequency hopping is one of the spread spectrum systems, and is a method for communication by specifying a rule called a hopping sequence between the transmitting side and the receiving side and switching a frequency of a carrier by predetermined time within a certain communication band according to the hopping sequence. In the MB-OFDM system, the preamble signal is also transmitted by the frequency hopping.
FIG. 9 shows a transmission example of the preamble signal in the MB-OFDM system.
The preamble signal is composed of 24 symbols S0 to S23 and transmitted through hopping three frequency bands f1 to f3. The receiving side synchronizes a receiving band with the frequency hopping in accordance with the hopping sequence, and thereby receiving and demodulating the symbols S0 to S23, which are dispersedly transmitted to the frequency bands f1 to f3. At the time of synchronizing, the hopping preamble signal must be surely detected. Therefore, the receiving side fixes the receiving band to a waiting band (the frequency band f1 in the example of FIG. 9), establishes symbol timing synchronization in the receiving period of a part of the symbols (the symbols S0 to S4 in the example of FIG. 9), and thereby detecting the preamble signal (hopping synchronization). After the preamble signal is detected, the frequency hopping is started, and in the receiving period of the remaining symbols (the symbols S5 to S23 in the example of FIG. 9), initial acquisition operations such as AGC (Automatic Gain Control), AFC (Automatic Frequency Control), and frame synchronization, are performed.
As described above, the symbol timing synchronization fixes the receiving band to one frequency band and is performed before AGC. Therefore, the detected symbol timing is not necessarily optimal for the received signal after AGC. In other words, the symbol timing synchronization is coarse timing synchronization. Therefore, after the symbol timing is adjusted in the operation period of AGC, it is usually necessary to perform processes such as AFC.
Japanese Unexamined Patent Application Publication No. 2007-19985 discloses such an adjusting method of the symbol timing. This adjusting method is to detect a difference between a peak position of a cross-correlation value and an expected value determined from a hopping timing while performing the frequency hopping and adjust the hopping timing to match the peak position of the cross-correlation value and the expected position. Since the hopping timing can be uniquely determined when the differences between the peak position and the expected position match in all the frequency bands, the symbol timing is used in common among the frequency bands. On the other hand, under the situation where the peak positions differ among the frequency bands, processes such as averaging and weighting are performed, and thereby compulsorily determining one symbol timing.
Note that as a related technique, Japanese Unexamined Patent Application Publication No. 2008-48239 discloses a technique to detect the symbol timing more accurately. Further, Japanese Unexamined Patent Application Publication No. 2006-74276 discloses the technique to reduce the power consumption at the time of detecting the symbol timing.
On the other hand, Japanese Unexamined Patent Application Publication No. 2009-141634 (Yasukawa) discloses a radio receiving apparatus that performs AFC. A configuration of the radio receiving apparatus 1x disclosed by Yasukawa is shown in FIG. 10.
The radio receiving apparatus 1x includes an RF (Radio Frequency) unit 10, an A/D (Analog to Digital) converting unit 20, a matched filter 30, an evaluating unit 40, a frequency correction circuit 50x, and a complex multiplier 60.
In the operation, the RF unit 10 receives a radio signal via an antenna in the state in which the receiving band is fixed to the waiting frequency band and converts the received signal into a complex baseband signal (I and Q signals). The A/D converting unit 20 includes an A/D converter to convert these complex baseband signals into digital signals. The matched filter 30 calculates a complex correlation value 401 from the digital signal output from the A/D converting unit 20 and the previously stored reference signal (known preamble signal pattern), and outputs the complex correlation value 401 to the evaluating unit 40.
The evaluating unit 40 detects a peak of the correlation value 401 and detects a detection timing thereof as a symbol timing 402. Then, the evaluating unit 40 repeatedly outputs the symbol timing 402 at a symbol period. In the MB-OFDM system, the symbol period is “165T” period as shown in FIG. 9. The period of “ 1/165 MHz” is indicated by 1T.
Further, when the symbol timing 402 is detected, the RF unit 10 starts the frequency hopping to the receiving band.
Furthermore, the frequency correction circuit 50x generates a frequency correction signal 403 according to the complex correlation value 401.
Specifically, the frequency correction circuit 50x is configured as shown in FIG. 11. The frequency correction circuit 50x includes “d” frequency offset detection circuits 310_1 to 310—d that are provided to correspond to the periods (d sample periods) in which a delayed wave could exist in the radio transmission line, an arctangent operator 320, an NCO (Numerically Controlled Oscillator) 350, an averaging unit 360, and “d−1” delay circuits 370_1 to 370—d−1.
Additionally, each of the frequency offset detection circuits 310_1 to 310—d includes an n-symbol delay circuit 311, a complex conjugate operator 312, and a complex multiplier 313.
In the operation, the complex correlation value 401 from the matched filter 30 is input to the frequency offset detection circuit 310_1 as it is. On the other hand, the correlation value 401 is delayed by one sample period and input by the delay circuits 370_1 to 370—d−1 to the frequency offset detection circuits 310_2 to 310—d. 
The n-symbol delay circuit 311 inside each frequency offset detection circuit 310_1 to 310—d delays the input complex correlation value 401 by the time equivalent to n symbols (n=“3” in the example of FIG. 9). The complex conjugate operator 312 performs a complex conjugate operation to the correlation value delayed by the n-symbol delay circuit 311.
The complex multiplier 313 performs complex multiplication to the complex correlation value input from the matched filter 30 and the complex conjugate operation result output from the complex conjugate operator 312. In other words, the complex multiplier 313 multiplies the correlation value corresponding to a current receiving symbol and a complex conjugate of the correlation value of n symbol before the current receiving symbol (the correlation value corresponding to the previous receiving symbol in the same receiving band).
Then, the averaging unit 360 calculates an average of the complex multiplication result respectively output from the frequency offset detection circuits 310_1 to 310—d. Further, the arctangent operator 320 performs an arctangent operation to the average result output from the averaging unit 360, and thereby obtaining a frequency offset. Moreover, the NCO 350 generates the frequency correction signal 403 for canceling the frequency offset output from the arctangent operator 320, and the above complex multiplier performs the complex multiplication
Accordingly, even when the symbol timing and the peak position of the correlation value shift as a result of AGC, the frequency offset between transmission and reception can be corrected.
In the MB-OFDM system in recent years, along with the expansion of the available carrier frequency bands, communication in high frequency bands in 6 GHz to 10 GHz is required in addition to the conventional 3 GHz to 4 GHz frequency bands. When the carrier frequency is increased, there is a problem that the propagation loss of the radio wave increases and the communication distance is reduced. Therefore, an improvement in the minimum receiving sensitivity in the radio receiving apparatus is desired.
On the other hand, with the cost-cutting demand in the market by the widespread use of mobile communication devices, cost reduction for LSI (Large Scale Integration) mounted on the radio receiving apparatus is also desired.